Overcurrent protection device

ABSTRACT

A two terminal circuit protection arrangement that: (1) is intended to be series connected in a line of the circuit; (2) comprises (a) a series switching transistor (1) that controls the line current, (b) a control transistor (4) that controls the base or gate voltage of the switching transistor and is responsive to an overcurrent through the switching transistor, and (c) a voltage source, for example a battery (3), a dc-dc converter (58/59) or a Seebeck device (43), applied to the base or gate of the switching transistor which biases the switching transistor into or toward conduction in normal operation; and (3) is capable of being remotely reset into a conducting state by stopping current in the line. The arrangement enables the initial voltage drop that is required to turn the switching transistor (1) on to be reduced or eliminated while requiring relatively little current from the voltage source. The arrangement can be reset remotely by briefly removing the voltage source or load from the circuit.

This invention relates to arrangements for protecting electricalcircuits from overcurrents, for example from overcurrents caused byequipment faults, electrostatic discharge or other threats, and tocircuits thus protected.

One circuit protection arrangement of relatively simple form isdescribed in German Patent Application No. 37 25 390 dated Jul. 31, 1987to Wickmann-Werke GmbH. This arrangement comprises a series switchingtransistor that controls the circuit current and a control transistorthat controls the base or gate voltage of the switching transistor. Thebase or gate voltage of the control transistor is set by a voltagedivider that spans the switching transistor, so that, if the arrangementexperiences an overcurrent, the control transistor will be biased intoconduction and will turn the switching transistor off. Although thisarrangement is particularly simple, it suffers from the disadvantagethat in normal operation there will always be a significant voltage dropacross the arrangement before it will conduct current, this voltage dropbeing due to the base-emitter junction voltage of the switchingtransistor added to the voltage drop across the base resistor in thecase of bipolar arrangements. In the case of arrangements based onenhancement mode FETs, the voltage drop will be due to the thresholdvoltage of the switching transistor. The voltage drop prevents this formof circuit protection arrangement being used in a number of applicationsand can lead to heat generation problems in high current applications.

Another circuit protection arrangement is described in German patentapplication No. 37 05 177 dated Feb. 18, 1987 to Siemens AG. Thisarrangement includes a power MOSFET in a line of the circuit, the gateof the MOSFET being biased on by a battery. A thyristor is connectedbetween the gate of the transistor and the circuit line on the sourceside of the transistor and senses the voltage drop in the line across aresistor connected in series with the MOSFET. When the voltage dropacross the resistor is greater than 0.7V the thyristor fires and theMOSFET is switched off. This circuit has the advantage that there is noinitial voltage drop before current can flow through the MOSFET.However, once the arrangement has switched into its blocking state itwill remain latched in that state and can be reset to its conductingstate only by means of a switch that shorts the anode and cathode of thethyristor. Furthermore, the battery needs to be able to supply a currentin the order of lmA to maintain the thyristor in its on state the wholetime from when the arrangement trips to its blocking state until it ismanually reset.

According to the present invention, there is provided a two terminalcircuit protection arrangement that: (1) is intended to be seriesconnected in a line of the circuit; (2) comprises: (a) a seriesswitching transistor that controls the line current; (b) and a controltransistor that controls the base or gate voltage of the switchingtransistor and is responsive to an overcurrent through the switchingtransistor; and (c) the arrangement including a voltage source appliedto the base or gate of the switching transistor which biases theswitching transistor into or toward conduction in normal operation; (3)is being capable of being remotely reset into a conducting state bystopping current in the line.

The arrangement according to the invention has the advantage that it ispossible to form an arrangement that reduces, or even eliminates theinitial voltage drop across the switching transistor before it conducts,while at the same time drawing a relatively low current from the voltagesource under all conditions. Furthermore, it is possible to reset thearrangement remotely for example by briefly removing the source or theload. As soon as the source or load is removed the arrangement willreset itself to its low resistance state.

Because the initial voltage drop can be reduced or eliminated it ispossible, in normal operation, for the only voltage drop across theswitching transistor to be due to its collector resistance or itschannel resistance. The voltage source may have any value up to or evenhigher than that required to bias the switching transistor intoconduction, the initial voltage drop across the switching transistorreducing as the voltage source potential rises.

The voltage source may be provided by any of a number of devices theparticular choice depending on a number of factors including the currentthat will be drawn from the voltage source. For example, it may comprisea battery. The battery will be in series with the control transistor anda resistor (the value of which determines at least partly the leakagecurrent of the arrangement). This resistor can have a relatively highvalue, for example 1 MΩ or more, often 10 MΩ or more, in which case themaximum current that will be drawn from the battery will be in the orderof 5 μA and preferably in the order of 500 nA or less when thearrangement has tripped. In the normal state of the arrangement when thecontrol transistor is off the current drawn from the battery willusually be in the order of picoamps, eg. less than 100 pA. Thus, thebattery may be formed as a small lithium cell having only a very smallcapacity, eg. in the order of lmAh which can be incorporated into anintegrated circuit package and will have a lifetime of a number ofyears. The voltage source is preferably connected in series with acurrent limiting resistor, especially where a battery is employed, inorder to prevent discharging of the battery when an overcurrent isexperienced and the control transistor is turned on.

Another form of voltage source that may be employed is a thermoelectricdevice such as a Seebeck device. Such a device is advantageously locatedin thermal contact with the switching transistor so that heat generatedby the switching transistor flows through the device. This arrangementhas the advantage that the thermoelectric device provides a feedbackarrangement in which an increase in heat generation in the switchingtransistor caused by the voltage drop across the switching transistorincreases the base or gate offset voltage and so reduces the voltagedrop. For relatively low frequency changes in the circuit current, thisfeedback can effectively reduce the switching transistor channelresistance.

Yet another voltage source that can be used is a dc-dc voltageconverter. Such converters are two-port networks which take a lowvoltage dc input and produce a higher dc voltage output. The convertermay be used to increase the voltage from another voltage source such asa Seebeck device mentioned above, or it may be connected across theswitching transistor so that the voltage drop across the switchingtransistor is multiplied and fed into its base or gate.

Other forms of voltage source that may be employed include photovoltaicdevices and capacitors that are charged up, for example by voltagemultiplication or by top-up charging when the switching transistor isoff. Alternatively, a separate supply may be employed for the voltagesource, for example a rectified mains supply.

If desired the base or gate voltage of the control transistor may bedetermined by a voltage divider that spans the switching transistor sothat the trip current of the arrangement is determined by the switchingtransistor channel resistance and the proportion of the voltage dropacross the switching transistor that is fed into the gate of the controltransistor. Alternatively, the base or gate of the control transistormay be connected directly to the collector or drain of the switchingtransistor so that the arrangement will trip if the overcurrent voltagedrop across the switching transistor exceeds the turn-on voltage of thecontrol transistor.

Where the arrangement is intended to be employed with ac circuits, itmay be connected to the line via a rectifying bridge circuit.Alternatively a pair of equivalent circuit protection arrangementsaccording to the invention may be employed, the two arrangementshandling different cycles of the ac signal. This latter arrangement hasthe advantage that the overall voltage drop across the arrangement isreduced due to a reduction in the number of diodes employed.

The overcurrent protection arrangement may employ either bipolarjunction transistor or field effect transistors, although FETs arepreferred since a bipolar switching transistor will require asignificant base current to be provided by the voltage source. Inaddition, the term "transistor" includes circuit elements employing morethan one transistor that can emulate the switching properties of atransistor, for example a number of transistors in a Darlingtonconfiguration. In the case of bipolar arrangements, Darlingtonconfigurations are preferred in order to reduce the switching transistorbase current. Not only does this base current load the voltage source,but it must be supplied via a resistor connected between the base andcollector of the switching transistor. When the circuit switches to itsblocking state the switching transistor base current is diverted throughthe control transistor (which is now on) and becomes a leakage current.However, since the voltage drop across the resistor is much higher whenthe arrangement is in its blocking state, the leakage current is largerthan the switching transistor base current. If a Darlington pair ortriplet is employed as the switching transistor, the effective dccurrent gain will be increased considerably so that a much higherresistance can be used.

A bipolar control transistor may advantageously be employed inconjunction with a field effect switching transistor. This arrangementhas the advantage that the trip voltage is reduced to a pn junctionvoltage drop, thereby allowing a switching transistor having a lowerchannel resistance to be used with a consequent reduction of powerdissipation.

Where field effect transistors are employed, enhancement mode MOSFETsshould be employed. The arrangement may be produced as an integratedcircuit, in which case the resistors employed in the switching circuitmay be provided by MOSFETs, for example with their gates and drainsconnected as in nMOS logic. Alternatively, the current limiting resistormay be replaced by a further FET that forms a complementary pair withthe control transistor.

According to a further aspect, the invention provides an electricalcircuit which comprises a circuit voltage or current supply, a load anda current-carrying line connecting the supply and load, the circuitincluding a two terminal circuit protection arrangement that: (1) isseries connected in the current-carrying line; (2) comprises: (a) aseries switching transistor that controls the line current; (b) acontrol transistor that controls the base or gate voltage of theswitching transistor and is responsive to an overcurrent through theswitching transistor; and (c) a voltage source applied to the base orgate of the switching transistor which biases the switching transistorinto or toward conduction in normal operation; and (3) is capable ofbeing remotely reset into a conducting state by removing the circuitvoltage or current source or the load.

Several forms of arrangement in accordance with the present inventionwill now be described by way of example with reference to theaccompanying drawings in which:

FIGS. 1 and 7 are circuit diagrams of the circuit protection arrangementaccording to the invention;

FIG. 2 is a graph showing the I-V characteristics of the circuit of FIG.1 and of the prior art (DE 37 25 390);

FIG. 3 is a circuit diagram of an alternative form of arrangementaccording to the invention;

FIGS. 4 and 5 are circuit diagrams of further forms of arrangementaccording to the invention that employ different voltage sources; and

FIG. 6 is a diagram of a modification of the circuit of FIG. 5.

Referring to the accompanying drawings, the circuit of a two-terminalarrangement for protecting a circuit from an overcurrent is shown inFIG. 1. The arrangement comprises an n-channel enhancement modeswitching MOSFET 1 that is connected between the terminals 2 and 2' ofthe device so that it passes the entire circuit current. The gate ofswitching transistor 1 is connected to its drain via battery 3 whichoffsets the gate voltage from the drain by the battery voltage andcurrent limiting resistor 5.

An enhancement mode control MOSFET 4 is connected across the gate sourcejunction of the switching transistor 1 in order to switch the switchingtransistor off when an overcurrent is experienced, the gate of thecontrol transistor being connected directly to the drain of theswitching transistor 1.

In normal operation of the circuit, if there is no current in the line,the switching transistor 1 will be on or off depending on whetherbattery 3 offsets the transistor gate by more or less than the thresholdvoltage of the transistor. When the line is loaded the voltage acrossthe switching transistor 1 will increase as the current increases asshown in FIG. 2 curve A, the slope of the curve depending on theswitching transistor channel resistance. The voltage will continue torise with increasing loading of the circuit until the trip voltage V_(T)is reached at which point drain source voltage of the switchingtransistor 1 is equal to the threshold voltage of the control transistor4, and the control transistor "shorts" the gate and source terminals ofthe switching transistor.

Once the arrangement has switched it will remain latched in its highresistance state even after the overcurrent has subsided because theresistance of transistor 1 is such that the entire circuit voltage isdropped across it. Thus, the arrangement must be disconnected from thecircuit supply or load before it will reset itself. Current limitingresistor 5 prevents rapid draining of the battery 3 when the arrangementhas tripped.

By way of comparison the I-V characteristic of a protection arrangementin accordance with German Application No. P 37 25 390 (employing FETs)is shown as curve B. This curve has the same form as curve A but isoffset to a higher voltage drop. This is due to the fact that an initialvoltage drop V_(I) must occur across the switching transistor before thegate of the control transistor reaches the threshold voltage. The I-Vcharacteristic of a protection arrangement in accordance with GermanApplication No. P 37 05 177 is similar to that of the present invention(curve A) until an overcurrent occurs, whereupon the device latches inits high resistance state with the I-V curve lying on the voltage axisuntil manually reset.

FIG. 3 shows an alternative form of two terminal protection arrangementin which an enhancement mode MOSFET 1 passes the operating current ofthe circuit. The gate of switching transistor 1 is connected to itsdrain via a battery 3 (eg. about 1.5V) and 1 Mohm current limitingresistor 5, and a control MOSFET 4 is connected across the gate-sourcejunction of transistor 1, as described with reference to FIG. 1.However, in this circuit the gate voltage of the control transistor 4 isheld by a voltage divider formed from 1 Mohm resistance 6 and 1.22 Mohmresistance 7 which span the switching transistor. In operation thisarrangement will perform in the same manner as that shown in FIG. 1 withthe exception that the magnitude of the current required to cause it toswitch is determined by the potential divider resistors 6 and 7 inaddition to the threshold voltage of transistor 4 and channel resistanceof switching transistor.

If desired, the battery can be connected between the gate and source ofthe switching transistor provided its polarity is changed.

FIG. 4 shows a similar arrangement to that shown in FIG. 3 in which thegate voltage of switching transistor 1 is controlled by controltransistor 4 whose gate voltage is set by a voltage divider formed fromresistors 6 and 7 that span the switching transistor 1.

In this arrangement the gate of the switching transistor is connected toits drain via a Seebeck device 43 that is in thermal contact with theswitching transistor 1 so that any heat generated in the switchingtransistor will cause a temperature difference between the junctions ofthe Seebeck device.

In operation, when current first flows along the circuit line, theswitching transistor is cold and no voltage is generated by the Seebeckdevice 43 so that an initial voltage drop of 1 to 2 volts occurs acrossthe switching transistor 1 as shown in FIG. 2 curve B. However, the heatgenerated in the switching transistor 1 by virtue of this voltage dropwill cause a voltage to be generated by the Seebeck device 43 which willbias the switching transistor's gate toward its drain and so reduce thevoltage drop across the switching transistor. Thus, a feedback mechanismis established that reduces power dissipated in the switchingtransistor.

As with the arrangement shown in FIG. 3, if an overcurrent occurs thearrangement will switch to its non-conducting state when the voltagedrop across the switching transistor 1 is sufficient to raise the gatesource voltage of transistor 4 to its threshold value.

In an alternative arrangement the output of the Seebeck device may beconnected in parallel with resistor 5, which is preferably being usedwith a bipolar switching transistor so that sufficient initial basecurrent can be provided. The Seebeck output may instead be connectedbetween the gate and source of the switching transistor.

FIG. 5 shows yet another form of arrangement according to the invention.In this arrangement switching transistor 1 is series connected in a lineof the circuit and its gate and source are connected together viacontrol transistor 4.

A dc-dc converter is to be included in order to convert 71 a low voltageappearing across the switching transistor 1 or part of that voltage, toa higher voltage to offset the gate of switching transistor 1. The input58 for the dc-dc converter 71 is between resistor 57 and the source ofthe transistors 1 and 4, and the output of the dc-dc converter 71 is tobe connected at 59, namely between the gate and source of the switchingtransistor 1, in series with current-limiting resistor 60. It is quitepossible, however, to connect the output between the gate and drain ofthe switching transistor 1.

In use, any voltage that appears across the switching transistor 1 willbe multiplied and fed back by the dc-dc converter 71 to offset the gateof switching transistor 1. This has the effect that for all currents upto the trip current the switching transistor has a relatively constantlow resistance and has an initial voltage drop (V_(I) in FIG. 2) of zerovolts. Resistor 60 prevents the control transistor 4 in its on statefrom loading the output 58 for the dc-dc converter 71.

If the arrangement is subject to an overcurrent, it will trip into itshigh resistance state when the voltage drop across the switchingtransistor 1' causes the gate source voltage of control transistor 4 torise to its threshold value, whereupon the current flowing through theswitching transistor 1 falls to substantially zero.

This form of arrangement has the advantage that all the componentsemployed are relatively reliable and do not need to be replaced, and inaddition, all the components are capable of integration to form amonolithic device.

Some dc-dc converters will require a short period of time to generate avoltage output after experiencing a current input. This delay will meanthat the control transistor 4 will switch on before the switchingtransistor 1 and so short out the source gate junction of the switchingtransistor, thereby causing the circuit to latch in its tripped state assoon as it is switched on. This problem may be overcome by thearrangement as shown in FIG. 6 which incorporates a start-up circuitcomprising a FET 60 that is connected across the source and gateterminals of the control transistor 4 and whose gate is held in an RCvoltage divider formed by capacitor 61 and resistor 62. When the circuitcurrent is switched on the RC voltage divider acts as a differentiator,causing the gate of FET 60 immediately to go high and then to fall toits source voltage as capacitor 61 charges. FET 60 will thereforeinitially be on, forcing the control transistor 4 to be off while thedc-dc converter begins to operate.

As shown in FIG. 6 the input for the dc-dc converter is regulated by aZener diode 63, and a resistor 64 of typically 500Ω will limit the inputcurrent to about 2 mA. Alternatively a current limiting diode circuitmay be employed in place of the resistor 64 and Zener diode 63.

What is claimed is:
 1. A two terminal circuit protection arrangementwhich:(1) is intended to be series connected in a line of a circuit tobe protected; (2) comprises:(a) a series switching transistor whichi. isselected from bipolar transistors and field effect transistors and(i) ifit is a bipolar transistor, comprises a collector, an emitter and abase, and (ii) if it is a field effect transistor, comprises a drain, asource and a gate, and ii. controls the line current; (b) a voltagedivider which is connected to the source and drain or to the emitter andcollector of the switching transistor, and (c) a control transistorwhichi. is selected from bipolar transistors and field effecttransistors, and(i) if it is a bipolar transistor, comprises acollector, an emitter and a base, and (ii) if it is a field effecttransistor, comprises a drain, a source and a gate, ii. has its base orgate voltage determined by the voltage divider, iii. controls the baseor gate voltage of the switching transistor, and iv. is turned on by avoltage across the collector and emitter or source and drain of theswitching transistor resulting from an overcurrent through the switchingtransistor, thereby causing the switching transistor to put thearrangement in a non-conducting state, and (d) a voltage source which isapplied to the base or gate of the switching transistor and which biasesthe switching transistor into or toward conduction in normal operation;and (3) is capable of being reset from the non-conducting state into aconducting state by a remotely controlled interrupting means whichinterrupts current in the line of the circuit to be protected.
 2. Anarrangement as claimed in claim 1, wherein the switching transistor andthe control transistor are each an enhancement mode field effecttransistor.
 3. An arrangement as claimed in claim 1, wherein the controltransistor is a bipolar transistor and the switching transistor is afield effect transistor.
 4. An arrangement as claimed in claim 1, whichincludes no resistive components in series with the switchingtransistor.
 5. An arrangement as claimed in claim 1, which includes acurrent limiting resistor in series with the voltage source.
 6. Anarrangement as claimed in claim 1, wherein the voltage source comprisesone or more electrical cells.
 7. An arrangement as claimed in claim 1,wherein the voltage source comprises a Seebeck device that is in thermalcontact with the switching transistor.
 8. An arrangement as claimed inclaim 1, wherein the switching transistor or the control transistor isan enhancement mode field effect transistor.
 9. A two terminal circuitprotection arrangement which:(1) is intended to be series connected in aline of a circuit to be protected; (2) comprises:(a) a series switchingtransistor whichi. is selected from bipolar transistors and field effecttransistors and(i) if it is a bipolar transistor, comprises a collector,an emitter and a base, and (ii) if it is a field effect transistor,comprises a drain, a source and a gate, and ii. controls the linecurrent; (b) a voltage divider which is connected to the source anddrain or to the emitter and collector of the switching transistor, and(c) a control transistor whichi. is selected from bipolar transistorsand field effect transistors, and(i) if it is a bipolar transistor,comprises a collector, an emitter and a base, and (ii) if it is a fieldeffect transistor, comprises a drain, a source and a gate, ii. has itsbase or gate voltage determined by the voltage divider, iii. controlsthe base or gate voltage of the switching transistor, and iv. is turnedon by a voltage across the collector and emitter or source and drain ofthe switching transistor resulting from an overcurrent through theswitching transistor, thereby causing the switching transistor to putthe arrangement in a non-conductive state, and (d) a voltage source,comprising a voltage multiplier whose input is taken across theswitching transistor, which is applied to the base or gate of theswitching transistor and which biases the switching transistor into ortoward conduction in normal operation; and (3) is capable of being resetfrom the non-conducting state into a conducting state by a remotelycontrolled interrupting means which interrupts current in the line ofthe circuit to be protected.
 10. A two terminal circuit protectionarrangement which:(1) is intended to be series connected in a line of acircuit to be protected; (2) comprises:(a) a series switching transistorwhichi. is selected from bipolar transistors and field effecttransistors and(i) if it is a bipolar transistor, comprises a collector,an emitter and a base, and (ii) if it is a field effect transistor,comprises a drain, a source and a gate, and ii. controls the linecurrent; (b) a voltage divider which is connected to the source anddrain or to the emitter and collector of the switching transistor, and(c) a control transistor whichi. is selected from bipolar transistorsand field effect transistors, and(i) if it is a bipolar transistor,comprises a collector, an emitter and a base, and (ii) if it is a fieldeffect transistor, comprises a drain, a source and a gate, ii. has itsbase or gate voltage determined by the voltage divider, iii. controlsthe base or gate voltage of the switching transistor, and iv. is turnedon by a voltage across the collector and emitter or source and drain ofthe switching transistor resulting from an overcurrent through theswitching transistor, thereby causing the switching transistor to putthe arrangement in a non-conductive state, and (d) a voltage source,comprising a voltage multiplier whose input is taken from one or moreelectrical cells or a Seebeck device, which is applied to the base orgate of the switching transistor and which biases the switchingtransistor into or toward conduction in normal operation; and (3) iscapable of being reset from the non-conducting state into a conductingstate by a remotely controlled interrupting means which interruptscurrent in the line of the circuit to be protected.
 11. An electricalcircuit which comprises a circuit voltage or current source, a load anda current-carrying line connecting the source and load, the circuitincluding a two terminal circuit protection arrangement which:(1) isseries connected in the current-carrying line; (2) comprises:(a) aseries switching transistor whichi. is selected from bipolar transistorsand field effect transistors and(i) if it is a bipolar transistor,comprises a collector, an emitter and a base, and (ii) if it is a fieldeffect transistor, comprises a drain, a source and a gate, and ii.controls the line current; (b) a voltage divider which is connected tothe source and drain or to the emitter and collector of the switchingtransistor, and (c) a control transistor whichi. is selected frombipolar transistors and field effect transistors and(i) if it is abipolar transistor comprises a collector, an emitter and a base, and(ii) if it is a field effect transistor comprises a drain, a source anda gate, ii. has its base or gate voltage determined by the voltagedivider, iii. controls the base or gate voltage of the switchingtransistor, and iv. is turned on by a voltage across the collector andemitter or source and drain of the switching transistor resulting froman overcurrent through the switching transistor, thereby causing theswitching transistor to put the arrangement in a non-conducting state,and (d) a voltage source applied to the base or gate of the switchingtransistor which biases the switching transistor into or towardconduction in normal operation; and (3) is capable of being reset fromthe non-conducting state into a conducting state by a remotelycontrolled interrupting means which disconnects the circuit voltage orcurrent sources, or the load, from the electrical circuit.
 12. Theelectrical circuit as claimed in claim 11, wherein the switchingtransistor and the control transistor are each an enhancement mode fieldeffect transistor.
 13. The electrical circuit as claimed in claim 11,wherein the control transistor is a bipolar transistor and the switchingtransistor is a field effect transistor.
 14. The electrical circuit asclaimed in claim 11, wherein the switching transistor or the controltransistor is an enhancement mode field effect transistor.
 15. Theelectrical circuit as claimed in claim 11, wherein the circuitprotection arrangement includes no resistive components in series withthe switching transistor.
 16. The electrical circuit as claimed in claim11, wherein the voltage source comprises a voltage multiplier whoseinput is taken across the switching transistor.
 17. A two terminalcircuit protection arrangement which:(1) is intended to be seriesconnected in a line of a circuit to be protected; (2) comprises:(a) aseries switching transistor whichi. is selected from bipolar transistorsand field effect transistors and(i) if it is a bipolar transistor,comprises a collector, an emitter and a base, and (ii) if it is a fieldeffect transistor, comprises a drain, a source and a gate, and ii.controls the line current; (b) a control transistor whichi. is selectedfrom bipolar transistors and field effect transistors, and(i) if it is abipolar transistor, comprises a collector, an emitter and a base, and(ii) if it is a field effect transistor, comprises a drain, a source anda gate, ii. controls the base or gate voltage of the switchingtransistor, and iii. is turned on by a voltage across the collector andemitter or source and drain of the switching transistor resulting froman overcurrent through the switching transistor, thereby causing theswitching transistor to put the arrangement in a non-conducting state,and (c) a voltage source whichi. comprises a voltage multiplier whoseinput is taken across the switching transistor, and ii. is applied tothe base or gate of the switching transistor and which biases theswitching transistor into or toward conduction in normal operation; and(3) is capable of being reset from the non-conducting state into aconducting state by a remotely controlled interrupting means whichinterrupts current in the line of the circuit to be protected.
 18. Thecircuit protection arrangement as claimed in claim 17, wherein theswitching transistor and the control transistor are each an enhancementmode field effect transistor.
 19. The circuit protection arrangement asclaimed in claim 18, wherein the gate of the control transistor isconnected directly to the drain of the switching transistor so that thearrangement will trip if the overcurrent voltage drop across theswitching transistor exceeds the threshold voltage of the controltransistor.
 20. The circuit protection arrangement as claimed in claim17, wherein the base or gate voltage of the control transistor isdetermined by a voltage divider which spans the switching circuit. 21.The electrical circuit as claimed in claim 17, wherein the controltransistor is a bipolar transistor and the switching transistor is afield effect transistor.
 22. The electrical circuit as claimed in claim17, wherein the switching transistor or the control transistor is anenhancement mode field effect transistor.
 23. The electrical circuit asclaimed in claim 17, wherein the circuit protection arrangement includesno resistive components in series with the switching transistor.
 24. Anelectrical circuit which comprises a circuit voltage or current source,a load and a current-carrying line connecting the source and load, thecircuit including a two terminal circuit protection arrangementwhich:(1) is series connected in the current-carrying line; (2)comprises:(a) a series switching transistor whichi. is selected frombipolar transistors and field effect transistors and(i) if it is abipolar transistor, comprises a collector, an emitter and a base, and(ii) if it is a field effect transistor, comprises a drain, a source anda gate, and ii. controls the line current; (b) a control transistorwhichi. is selected from bipolar transistors and field effecttransistors and(i) if it is a bipolar transistor, comprises a collector,an emitter and a base, and (ii) if it is a field effect transistor,comprises a drain, a source and a gate, and ii. controls the base orgate voltage of the switching transistor, and iii. is turned on by avoltage across the collector and emitter or source and drain of theswitching transistor resulting from an overcurrent through the switchingtransistor, thereby causing the switching transistor to put thearrangement in a non-conducting state, and (c) a voltage source whichi.comprises a voltage multiplier whose input is taken across the switchingtransistor, and ii. is applied to the base or gate of the switchingtransistor and which biases the switching transistor into or towardconduction in normal operation; and (3) is capable of being reset fromthe non-conducting state into a conducting state by a remotelycontrolled interrupting means which disconnects the circuit voltage orcurrent source, or the load, from the electrical circuit.
 25. Thecircuit protection arrangement as claimed in claim 24, wherein theswitching transistor and the control transistor are each an enhancementmode field effect transistor.
 26. The circuit protection arrangement asclaimed in claim 25, wherein the gate of the control transistor isconnected directly to the drain of the switching transistor so that thearrangement will trip if the overcurrent voltage drop across theswitching transistor exceeds the threshold voltage of the controltransistor.
 27. The circuit protection arrangement as claimed in claim24, wherein the base or gate voltage of the control transistor isdetermined by a voltage divider which spans the switching circuit. 28.The electrical circuit as claimed in claim 24, wherein the controltransistor is a bipolar transistor and the switching transistor is afield effect transistor.
 29. The electrical circuit as claimed in claim24, wherein the switching transistor or the control transistor is anenhancement mode field effect transistor.
 30. The electrical circuit asclaimed in claim 24, wherein the circuit protection arrangement includesno resistive components in series with the switching transistor.